The invention relates to a scanner for a clock disc including light transmitting slits which are uniformly distributed over the circumference of the clock disc for transmitting signals to an evaluation circuit, particularly to control the position of a driven member operated by a d.c. motor, e.g. a rotatably mounted printed wheel in a typewriter or office machine of similar construction, wherein the clock disc is fixed to a drive shaft of the motor and the slits can be scanned by means of a light barrier arrangement in the form of a position sensor including a light source for projecting a bundle of light beams along respective beam paths and a light receiver disposed for receiving the light beams after having passed through the slits and producing output voltages representing the received light beams.
Scanners are employed, for example, for incremental and/or absolute digital length or angle measuring systems in which photo elements convert a light current modulated by the gradations of a scale and a scanning plate into electrical signals which serve to determine and digitally display path informations.
It is known, for example, to determine the position of the printing mechanism relative to the platen and the record carrier by optically scanning the slits in a scanning disc fastened to the motor shaft of the driving d.c. motor. For an unequivocal position determination, two incremental scanners are normally required which, upon rotation of the shaft, emit two pulse signals which are electrically shifted in phase by 90.degree.. To determine the position of the printing mechanism, the pulses supplied by the optical scanners are summed, namely with a positive sign in the one direction of movement of the printing mechanism and with a negative sign in the other. The positive or the negative sign are here derived from the phase position of the two pulse signals. The scanners employed in this arrangement are adjusted electrically which is relatively complicated.
Moreover, U.S. Pat. No. 4,270,868 discloses a digital control device for a d.c. motor in printers in which a scanning disc fastened on the shaft of the d.c. motor generates, in conjunction with a detector, produces a series of pulses whose succession in time is inversely proportional to the number of revolutions of the d.c. motor. The time between discrete positions of the motor shaft is compared with a desired time and the difference is stored in a register. This difference is converted to a pulse width modulated signal which directly controls the number of revolutions of the motor.